It reveals the killer, which is in Season 2. And the movie makes a few allusions to the second season you might not get. But otherwise I don't think it spoils much. Frankly, I think the film is a world apart from the series. It's everything you knew about Laura Palmer prior to her murder, but under an intense microscope that shows all the seedy shit you can't get into on TV. A far darker movie with almost none of the humor or lightness of the show.
They reveal the killer in the middle of the season due to pressure from Mark Frost and the producers. So yeah, that kind of cuts it off and ends up spiralling into a bunch of disjointed strangeness. Like, random strange rather than good Lynchian strange (sans the final episode, which is brilliant).
If you already have a good feeling for who the killer is, you can watch it.
I've always thought it was her dad. Don't tell me if I'm right or wrong, it's just a hunch I've had.
But yeah, I may as well see it, I want to see more Lynch flicks anyway and I think that's the only one that's really easy to find that I haven't picked up yet.